1. Field of the Invention
This invention relates to modems, and in particular to systems and methods for reducing the processing requirements of a modem.
2. Description of the Related Art
Modems are communications devices which employ digital modulation techniques to transmit binary data over analog communications channels, e.g., telephone lines. Typically, two modems communicate over a single channel, with one modem at each end of the channel. Signal processing structures implemented at each modem provide encoding, modulation, filtering, interpolation, echo cancellation, signal detection, equalization, demodulation, and decoding functionality. Modems typically conform to international standards to ensure interoperability with modems from other manufacturers. One such standard is the V.34 specification described in V.34, A Modem Operating at Data Signalling Rates of up to 28 800bits/s for Use on the General Switched Telephone Network and on Leased Point-to-Point 2-Wire Telephone-Type Circuits, dated September, 1994 (previously CCITT Recommendation V.34), which is hereby incorporated herein, in its entirety, by reference.
Traditional modem implementations include one or more dedicated digital signal processors (or DSPs) on which signal processing algorithms execute during periods of modem operations. A computer system may incorporate such a modem implementation, and in addition, typically includes application and operating system software executable on a general purpose processor (e.g., any of a variety of processors including MIPS(trademark) R3000, R4000, and R5000 processors, processors conforming to the SPARC(trademark), PowerPC(trademark), Alpha(trademark), PA-RISC(trademark), or x86 processor architectures, etc.). Software executing on the general purpose processor sends and receives data via the modem implementation using input/output (I/O) ports, direct memory access (DMA), or other I/O structures and methods suitable for a particular general purpose processor and operating system combination.
Since a typical modem implementation includes a dedicated DSP not shared with other signal processing functions of a larger computer system, the modem""s DSP and the signal processing algorithms designed to run thereon are selected and designed to meet the peak computation load of the modem. DSP cycles are either used or lost. For this reason, signal processing algorithms implementing the complete steady-state functionality of modem transmit and receive paths are typically executed on a DSP at full speed for the duration of a modem connection.
For many portable device applications such as Personal Digital Assistants (PDAs), portable computers, and cellular phones, power consumption, battery life, and overall mass are important design figures of merit. In addition, very small part counts are desirable for extremely-small, low-cost consumer devices. Modem communications are desirable in many such portable device applications. However, traditional DSP implementations of the underlying signal processing capabilities create substantial power demands, require increased part counts, and because of the power consumption of a discrete DSP, typically require larger, heavier batteries.
A modem implemented as software executable on a general purpose computer could reduce part counts, power demands, and overall mass of a computer system by eliminating the DSP, its power consumption, and some of the battery capacity otherwise required. However, to fully benefit from the elimination of a DSP, such a software modem needs to co-exist with existing operating systems and applications, running on the same general purpose processor as the operating systems and applications. Unfortunately, in a computer system which includes such a software modem, the load on the general purpose processor can be significant, slowing operating system and application programs, even when the software modem is not sending or receiving data. For example, FIG. 1 depicts a 14 MIPS steady-state computational load, which is exemplary of a software implementation of a traditional modem maintaining an active connection with both transmitter and receiver structures active for the duration of the connection.
The computational load imposed by communications software executed on a processor can be significantly reduced by exploiting periods during an active connection when no data is being received. In particular, execution of many receive path signal processing algorithms can be disabled when no data is being received. The transmit path continues output modulation as with a normal connection so as to trick a remote communications device into believing the connection is still normal. However, because substantial portions of the local receive path can be disabled, computational load on the processor is reduced and additional compute cycles are freed for application and/or operating system program use.
Disabled portions of the receive path are re-enabled (restarted) in response to wake up events. For example, receive path algorithms can be re-enabled under program control, e.g., when the user, application program, or operating system initiates data transmission, such as in response to a user selection of a new Universal Resource Locator (URL) within a World-Wide Web (WWW) browser application. Because many network-based applications, e.g., browsers, electronic mail clients and servers, message retrieval clients, netcasting receivers, etc., exploit a transactionbased model of interprocess communication, receive path algorithms can be enabled coincident with the start of such a transaction and disabled coincident with completion of the transaction. Receive path algorithms can also be periodically re-enabled, e.g., at programmable intervals. In this way, the receive path is periodically available for retransmission of data missed while the receive path algorithms were disabled (i.e., while the receiver was xe2x80x9casleep,xe2x80x9d or in xe2x80x9cdozexe2x80x9d mode). Higher level data protocols (e.g., TCP/IP) will retransmit any data that was missed while the receiver was xe2x80x9casleep.xe2x80x9d
In addition to program controlled and periodic wake up events, wake up events may be triggered by the remote communications device. For example, in a modem embodiment in accordance with ITU-T Recommendation V.34, remote retrain, remote rate renegotiation, and remote cleardown requests are received by an undisabled set of receive path signal processing algorithms. A wake up event is triggered in response to detection of a remote retrain, remote rate renegotiation, or remote cleardown request and disabled portions of the local modem""s receive path are re-enabled in response thereto.
By providing functionality (e.g., in the form of functions, procedures, and/or methods) for enabling and disabling portions of a software modem""s receive path algorithms, a software modem application program interface (API) in accordance with an embodiment of the present invention allows application-and operating system processes to switch between a doze mode (with low computational load) during idle receive time and a full operational mode (with full computational load) during active receive time. In this way, compute cycles otherwise allocated to idle-time execution of receive path algorithms are allocable to application program or operating system processes. In an exemplary implementation, computational requirements for the software modem can be reduced from 100% in steady-state, to less than 25% of the steady state load during idle receive time, i.e., when no data is being received by the software modem. In contrast, traditional approaches require the full 100% whether or not the software modem is receiving data.